I have several ideas why the VC goes in the HF2AV. My old VC will lock up when heated with a torch. The t-case would get really hot and lock the VC while driving at 65+mph on hot days. It even got hot enough to lock the VC with the front driveshaft off and the diff lock switch locked. This happened after I put MI 75w90 in it. That stuff is like water. I haven't had time to get a dino 75w90 to see how thick it is compaired to the M1. I think maybe the thinner M1 oil was to thin for my old 206k mi t-case and the bearings were not getting lubed enough and were getting hot enough to lock the VC. As it is NOT summer any more up here, I need a victim from the SW with a IR temp reader thingy. They need to drive at 70+ on a hot 80f+ day and take some temps. I will heat up my bad VC and take temps and see how hot it is when it locks. But I will need a victim to help me.